While a process according to the present invention is useful for the formation of a film of a fluorine-containing resin on any of various forms of metallic substrates, the advantages of the process, which are to be clarified as the description proceeds, will be exploited most effectively where the film to be formed on the substrate is required to provide high degrees of hardness, adhesion to substrate, non-stickiness, surface-to-surface fitness, smoothness, water repellency, wear and abrasion resistances, scratch resistance, corrosion resistance, and/or resistances to stress and strain. In the description to follow, the substrates which can be treated by a process according to the present invention will be exemplified by thermal fixation rolls of duplicating machines; castings, especially of aluminum and aluminum alloys, which are represented by certain kinds of cooking utensils or vessels to be subjected to heat such as kettles, pots and pans; certain kinds of machine components of precision, optical and other machines and equipment such as precision cutting knives or blades of metal and non-metal working machines and fins of heat exchangers; certain kinds of precision articles of metal such as needles of syringes; and component members of some machines and equipment such as soldering jigs and molds for forming thermoplastic and thermosetting plastics.
A thermal fixation roll of a duplicating machine is a hollow cylinder of metal used for the thermal fixation of tonor particles. It is constructed usually of an aluminum alloy, copper, or steel and ordinarily measures from about 30 cm to about 60 cm in diameter and from about 320 cm to about 400 cm in length as well known in the art. Such a thermal fixation roll of a duplicating machine typically has its outer peripheral surface uniformly coated with polytetrafluoroethylene or polytetrafluoroethylene/perfluoropropyl vinyl ether copolymer. Problems have thus far been pointed out in connection with the films of polytetrafluoroethylene and polytetrafluoroethylene/perfluoropropyl vinyl ether copolymer thus applied to the outer peripheral surfaces of thermal fixation rolls presently in use.
The film of, for example, polytetrafluoroethylene applied to the outer peripheral surface of a thermal fixation roll is acceptable in hardness but has a drawback in that, when the roll is to be coated with polytetrafluoroethylene with a film of silicon oil preliminarily applied to the surface of the roll, the silicon oil infiltrates into the porosities in the film and is caused to expand and form a myriad of swells or blisters on the surface of the roll when the roll is subjected to rapid heating. Formation of these swells or blisters seriously impairs the strength of adhesion of the film to the surface of the substrate and surface-to-surface fitness, smoothness, water repellency, wear and abrasion resistances, scratch resistance, and resistances to stress and strain of the film and results not only in shortened durability and accordingly in shortened lifetime of the roll per se but in deterioration in the performance quality of the duplicating machine using such a roll.
On the other hand, the coating of polytetrafluoroethylene/perfluoropropyl vinyl ether copolymer applied to the outer peripheral surface of a thermal fixation roll is a thin film of the thickness of from about 20 microns to about 40 microns having a significantly small number of porosities and is for this reason practically free from the problem encountered by the film of polytetrafluoroethylene. A drawback of the film of polytetrafluoroethylene/perfluoropropyl vinyl ether copolymer is that it has a degree of hardness corresponding to the grades B to 2B in terms of pencil lead hardness and is inferior in this respect to the film of polytetrafluoroethylene which has a degree of hardness corresponding to the grade F or HB of the pencil lead hardness although the hardness of each of the films may vary depending upon the thickness of the film. The coating of polytetrafluoroethylene/perfluoropropyl vinyl ether copolymer applied to a thermal fixation roll is thus unacceptable in hardness, adhesion to substrate, non-stickiness, surface-to-surface fitness, smoothness, wear and abrasion resistances, scratch resistance, and resistances to stress and strain and may also degrade the performance quality of the duplicating machine using the roll.
Numerous variants of a process of coating the surface of a thermal fixation roll with polytetrafluoroethylene, polytetrafluoroethylene/perfluoropropyl vinyl ether copolymer or a copolymer of polytetrafluoroethylene and polyhexafluoropropylene have therefore been proposed to provide solutions to these problems. Representative examples of these variant processes are disclosed in Japanese Provisional Patent Publications No. 54-127447, No. 54-149745, No. 56-93517 and No. 56-133770 which are herein cited as references. One of the processes taught in these published materials uses polytetrafluoroethylene as a primer over which particles of polytetrafluoroethylene/perfluoropropyl vinyl ether copolymer are applied to form a non-sticky film on the outer peripheral surface of a thermal fixation roll. For the formation of such a film, the outer peripheral surface of the roll to be processed is roughened in an appropriate manner and a primer of polytetrafluoroethylene is applied to the roughened outer surface of the roll. The roll thus having the powder of the polytetrafluoroethylene/perfluoropropyl vinyl ether copolymer applied to the film of the primer on the surface of the roll is heated to fuse the copolymer under atmospheric pressure or in a vacuumized furnace. During sintering of the coating of the polytetrafluoroethylene/perfluoropropyl vinyl ether copolymer, the primer underlying the coating being heated tends to produce stains on the surface of the coating and degrades the various properties of the resultant coating of the copolymer as well as the external appearance of the coated article. If an aminosilane is used as a primer to avoid formation of such stains on the coating of the polytetrafluoroethylene/perfluoropropyl vinyl ether copolymer, the resultant coating will fail to have a sufficient strength of adhesion to the surface of the roll which is to be frequently subjected to heat for an extended period of time.
On the other hand, the coatings of fluorine-containing resins on castings of aluminum and aluminum alloys to be used as some cooking vessels such as kettles, pots and pans are generally required to have high degrees of hardness, adhesion to substrate or to primer, non-stickiness, smoothness, wear and abrasion resistances, scratch resistance, corrosion resistance, and resistances to stress and strain. Various processes are known for the formation of coatings of fluorine-containing resins on castings in general of aluminum and aluminum alloys. These processes are largely broken down into two major categories: the processes using primers or undercoats and the processes forming the coatings of the fluorine-containing resins directly on the surfaces of the castings. In one of the processes of the latter type, the surface of a casting of aluminum or an aluminum alloy is etched in a solution of an inorganic acid such as typically hydrochloric acid, hydrofluoric acid or phosphoric acid or of a mixture of two or more of these at an ambient temperature or at an appropriately elevated temperature. The surface of the casting thus roughened by chemical etching is anodized to form an oxidized corrosion resistant layer thereon, whereupon a dispersed solution of a fluorine-containing resin is applied to the resultant surface of the casting. The casting is then heated to a temperature higher than the melting point of the fluorine-containing resin to form the desired film of the fluorine-containing resin on the roughened, corrosion resistant surface of the aluminum or aluminum-alloy casting. Another process of forming a film of a fluorine-containing resin on an aluminum or aluminum-alloy casting without use of a primer involves electrolytic etching in an aqueous solution of a halogenide such as typically sodium chloride or potassium chloride to have the surface of the casting roughened to a desired degree.
Having recourse to formation of an undercoat of polyhexafluoropropylene or aminosilane on the surface of the article to be coated with a fluorine-containing resin is objectionable since such a primer tends to produce stains on the surface of the coating or the coating formed on the primer tends to fail to have an adequate strength of adhesion to the surface of the substrate, as discussed previously.
Where a casting of an aluminum alloy is used as the substrate on which the film of a fluorine-containing resin is to be formed in any of the known processes dispensing with the use of primers, the proportion of the element aluminum in the alloy is short of assuring effective anodic oxidation of the casting so that it is difficult to form a sufficiently stabilized corrosion resistant layer in the surface of the casting to be coated with the fluorine-containing resin. This problem is more serious where the aluminum alloy used contains another alloy component such as silicon since the existence of such an additional alloy component promotes production of smuts on the surface of the casting upon acid pickling of the surface. Aluminum or aluminum alloys of only limited types have for these reasons been thus far used as the substrate on which a coating of a fluorine-containing resin is to be formed without using a primer.
During chemical or electrolytic etching of the surface of a casting of aluminum or an aluminum alloy as above discussed, the chemical etchant or the electrolytic solution is allowed to penetrate into the fine porosities or concavities formed in the roughened surface of the casting. The chemical solution thus trapped into these porosities or concavities in the casting can not be removed completely by mere repetition of rinsing in water. If the solution fails to be removed from the casting completely, the small quantities of the solution remaining in the porosities or concavities of the casting produce stains or discolored spots on the film of the fluorine-containing resin formed on the surface of the casting and also degrade the external appearance of the coating per se and accordingly the commercial value of the coated article. In the worst case, the presence of the chemical etchant or the electrolytic solution left in the porosities or concavities might result in formation of a layer of a hydroxide (which is usually called rust in the art) on the surface of the aluminum or aluminum-alloy casting. The formation of such a layer in turn results in shortened lifetime of the aluminum or aluminum-alloy article.
As noted previously, a process according to the present invention is applicable also to the formation of a coating of polytetrafluoroethylene on the surface of any of certain kinds of machine components of precision, optical and other machines and equipment such as precision cutting knives or blades of metal and non-metal working machines and fins of heat exchangers, certain kinds of precision articles of metal such as needles of syringes, and component members of some machines and equipment such as certain kinds of soldering jigs and molds for forming thermoplastic and thermosetting plastics. Various processes are also known for the formation of such coatings on substrates of, for example, aluminum, an aluminum alloy, mild steel, stainless steel or brass (a zinc-copper alloy). To form a film of polytetrafluoroethylene on the surface of such a substrate in one of the known processes, the substrate is preliminarily etched either chemically or electrolytically and a dispersed solution of polytetrafluoroethylene is applied to the surface thus roughened of the substrate and is heated at an atmospheric or subatmospheric pressure to form a hardened solid layer of polytetrafluoroethylene on the surface of the substrate. In a process using a primer, polyhexafluoropropylene, alkyl titanate or an aminosilane is applied to the roughened surface of the substrate whereupon a dispersed solution of polytetrafluoroethylene is applied to the primer thus formed preliminarily on the surface of the substrate. None of these processes is however fully acceptable for producing a uniform coating of polytetrafluoroethylene satisfactory for any of the above enumerated articles. Where the substrate to be coated is in the form of a cast product of, for example, an aluminum alloy which has an abundance of cavities or porosities in the surface layer, the coating may fail to be formed in the cavities or porosities and will, as a result produce irregularities of thickness throughout its area.
It is, accordingly, an important object of the present invention to provide an improved process of forming a film of a fluorine-containing resin on the surface of a metallic substrate to achieve increased degrees of hardness, adhesion to a substrate, non-stickiness, surface-to-surface fitness, smoothness, water repellency, wear and abrasion resistances, scratch resistance, corrosion resistance, and/or resistances to stress and strain.
It is another important object of the present invention to provide an improved process of forming a film of a fluorine-containing resin on the surface of a metallic substrate such as a thermal fixation roll of a duplicating machine, wherein the process is adapted to preclude formation of swells or blisters on the surface of the coating of the fluorine-containing resin when the coating on the substrate is subjected to rapid heating in the presence of silicon oil.
It is still another important object of the present invention to provide an improved process of forming a film of a fluorine-containing resin on the surface of a metallic substrate such as a thermal fixation roll of a duplicating machine, wherein the process is adapted to preclude production of stains on the surface of the coating during heating of the coating on the substrate.
It is still another important object of the present invention to provide an improved process of forming a film of a fluorine-containing resin on the surface of a metallic substrate without having recourse to the formation of a primer or undercoat on the surface of the substrate.
It is still another important object of the present invention to provide an improved process of forming a film of a fluorine-containing resin on the surface of a casting of, particularly, aluminum or an aluminum alloy, wherein the surface of the substrate to be coated with the fluorine-containing resin is mechanically roughened prior to being anodized to form an oxidized corrosion resistant layer so that there will be no chemical solution trapped in the porosities or concavities formed in the casting during the roughening step.
It is still another important object of the present invention to provide an improved process of forming a film of a fluorine-containing resin on the surface of a casting of aluminum or an aluminum alloy in such a manner that there will be produced on the film of the fluorine-containing resin no such stains or discolored spots that will degrade the external appearance of the coating per se and accordingly the commercial value of the coated article.
It is still another important object of the present invention to provide an improved process of forming a film of a fluorine-containing resin on the surface of a casting of aluminum or an aluminum alloy in such a manner that there will not be produced on the surface of the casting a layer of hydroxide which will contribute to reduction in the lifetime of the coated aluminum or aluminum-alloy article.
It is still another important object of the present invention to provide an improved process of forming a film of polytetrafluoroethylene on the surface of any of certain kinds of machine components of precision, optical and other machines and equipment such as precision cutting knives or blades of metal and non-metal working machines and fins of heat exchangers, certain kinds of precision articles of metal such as needles of syringes, and component members of some machines and equipment such as soldering jigs and molds for forming thermoplastic and thermosetting plastics.
It is still another important object of the present invention to provide an improved process of forming a satisfactorily uniform film of polytetrafluoroethylene on the surface of any of these precision members and articles.
Yet it is still another important object of the present invention to provide an improved process of forming a film of a fluorine-containing resin on the surface of a metallic substrate to achieve excellent mechanical properties of the coating and an excellent external appearance of the coated article.